Multiplex of high definition radio stations

ABSTRACT

A system for peak-to-average-power ratio (PAPR) reduction of a frequency shifted plurality of digital broadcast signals taking into account the combined signal peaks in order to transmit the signals more efficiently in a single broadcast transmission system. The PAPR algorithm takes into account a rotating constellation phase offset for the shifted signals corresponding to the amount of applied frequency shift. In the case of a dual sideband In-Band-On-Channel (IBOC) signal typically used in conjunction with an FM carrier in the center, the sidebands can be interleaved to create a new IBOC signal definition and take the place of the FM carrier for an all-digital transmission that is backward compatible with IBOC receivers allowing for a gradual migration to all digital broadcasting.

STATEMENT OF RELATED APPLICATION(S)

This application is a 35 U.S.C. § 371 national phase of InternationalApplication No. PCT/CA2016/050406 filed on Apr. 8, 2016, and claims thebenefit of Canadian Patent Application No. 2,887,751 filed on Apr. 10,2015. The entire disclosures of International Application No.PCT/CA2016/050406 and Canadian Patent Application No. 2,887,751 arehereby incorporated herein by reference herein in their respectiveentireties.

TECHNICAL FIELD

The current disclosure relates to the transmission of OFDM signals and,in particular, to multiplexing multiple stations together.

BACKGROUND

Radio broadcasts have transitioned from an all-analog signal to a hybridsignal combining both digital and analog signals. The digital signalsmay provide better efficiency, for example, allowing more audio streamsto be broadcast in the same bandwidth.

It would be desirable to be able to provide a system capable oftransmitting an all-digital signal that is compatible with at least aportion of current receivers.

SUMMARY

In accordance with the present disclosure there is provided a digitalbroadcast system comprising: an input component for receiving aplurality of stations and frequency-shifting the stations to provide amultiplex of the plurality of stations; a power reduction component forpeak reducing the multiplex and providing a corresponding output signal;and a transmitter component for transmitting the output signal.

In a further embodiment of the system, the peak reduction is based on aniterative clipping and correction algorithm that controls the in-bandand out-of-band noise from peak reduction.

In a further embodiment of the system, the correction algorithm operateson the signal in the frequency domain.

In a further embodiment of the system, the power reduction componentundoes a phase shift prior to frequency domain corrections andre-applies the phase shift following the frequency domain corrections.

In a further embodiment of the system, two or more of the plurality ofstations comprise IBOC stations.

In a further embodiment of the system, two or more of the plurality ofstations in the multiplex are interleaved in frequency

In a further embodiment of the system, two or more of the plurality ofstations comprise DRM+ stations.

In a further embodiment of the system, two or more of the plurality ofstations comprise China Digital Radio stations.

In a further embodiment of the system, two or more of the plurality ofstations in the multiplex are interleaved in frequency.

In a further embodiment of the system, service modes and sideband levelsof the IBOC stations are independently configurable.

In a further embodiment of the system, each of the plurality of stationsin the multiplex are independently adjustable in power.

In a further embodiment of the system, the plurality of stations areproduced by multiple independent modulators.

In a further embodiment of the system, each of the respective modulatorshave synchronized symbol timing.

In a further embodiment of the system, each of the respective modulatorsproduce OFDM with perfect frequency and standard subcarrier spacing.

In a further embodiment of the system, the frequency shift applied toeach station is an integral of the subcarrier spacing.

In a further embodiment of the system, one or more of the plurality ofstations use a single frequency network implementation.

In a further embodiment of the system, the output signal includes one ormore FM carriers.

In a further embodiment of the system, the one or more FM carriers allowa receiver to scan for one or more of the plurality of stations.

In a further embodiment of the system, the FM carrier is modulated toprovide a signal instructing a listener to tune in via a digital radio.

In a further embodiment, the system further comprises a second inputcomponent for receiving a second plurality of stations andfrequency-shifting the stations to provide a second multiplex of theplurality of stations that has synchronized symbol timing with themultiplex; and a second power reduction component for peak reducing thesecond multiplex and providing a corresponding second output signal.

In a further embodiment, the system further comprises a signal combinerfor combining the output signal with the second output signal fortransmission by the transmitter.

In a further embodiment, the system further comprises a secondtransmitter component for transmitting the second output signal.

In accordance with the present disclosure there is further provided adigital radio receiver capable of receiving a transmitted digitalmultiplex comprising a plurality of frequency shifted stations havingtime-aligned symbols and decoding data on one or more of the pluralityof frequency shifted stations.

In a further embodiment of the digital radio receiver, a single receiverstack is used in decoding the data.

In a further embodiment of the digital radio receiver, the receiverstack comprises one or more of: a symbol tracking loop; a channelestimator; and a data decoder that undoes a phase shift of symbols dueto the station frequency shift.

In a further embodiment of the digital radio receiver, the receiver isat least one of: an HD Radio receiver; a DRM receiver; and a ChinaDigital Radio receiver.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a three station multiplex;

FIG. 2 shows a block diagram of a multiplexing system;

FIG. 3 depicts a PAPR reduction processes;

FIG. 4 depicts 3 interleaved stations;

FIG. 5 depicts a 400 kHz mode;

FIG. 6 depicts an unmodulated FM Carrier;

FIG. 7 depicts a dual station configuration;

FIG. 8 depicts a 4 station configuration;

FIG. 9 depicts a 6 station configuration;

FIG. 10 depicts a multiplex having an FM channel;

FIG. 11 depicts a typical channel interleaving pattern;

FIG. 12 depicts a multiplex receiver implementation; and

FIG. 13 depicts a 4 station configuration over 1 MHz.

DETAILED DESCRIPTION

The HD Multiplex concept is an extension of the IBOC system. The outputof multiple independent IBOC exgine modulators can be combined in asingle crest factor reduction engine, such as that described in U.S.Pat. No. 6,128,350 of Shastri et al. and U.S. Pat. No. 8,369,431 ofWalker et al. (referred to as HD PowerBoost further herein) the contentsof which are incorporated herein by reference in their entirety. Thisallows a single multiplex of 2 or more IBOC stations to be amplifiedusing a single transmitter and subsequent antenna system by replacingthe FM signal used in hybrid IBOC transmission. This results in anall-digital IBOC configuration capable of carrying up to 3 times thestandard IBOC payload. Such a multiplex can carry up to 15 audio streamsin 600 kHz of signal bandwidth. Such channel multiplexing can beextended by adding further sidebands in various permutations. FIG. 1shows a three station multiplex with the three separate stations markedas A, B and C. The corresponding IBOC sidebands are all spaced thestandard 200 kHz apart and therefore maintain backward compatibilitywith existing receivers.

The described system provides an effective migration path from today'shybrid HD radio implementation, that places two IBOC sidebands, one oneach side of the traditional FM carrier, to an all-digital IBOC signal.This is backward compatible with a large cross section of the existingreceiver base and can coexist with standard FM stations. As describedfurther herein, a single broadcast transmitter and transmission systemcan be used to fill in the spectrum presently allocated to the FMcarrier with IBOC carriers from 2 or more independently modulated IBOCsignals. Modification to the established IBOC crest factor reductionoutlined by Shastri et al., which is also the basis of HD PowerBoost,allows the crest factor reduction to operate on all of the independentstations producing a signal with comparable peak to average power ratio(PAPR) to a present day IBOC only transmitter. The number of added IBOCsignals at the same power level can scale with transmitter size allowinga larger, more efficient, transmitter model to be used for all signals.Since a single IBOC signal only requires about 10% signal power toachieve FM comparable coverage, a three station multiplex as shown inFIG. 6 requires less than 30% of the power of a comparable FM carrier.

One example configuration uses 600 kHz of bandwidth that interleaves 3stations, identified as A, B and C, is shown in FIG. 1. Using theestablished service mode MP3, the effective IBOC bandwidth of 123.2 kBpscan be tripled to 369.6 kbps. Using a combination of 32 kbps, 24 kbpsand 14 kbps audio streams, up to 15 audio streams can be placed on thismultiplex. It is conceivable that up to 240 audio streams could beplaced in the FM band for a given market assuming a 50% band frequencyuse once the entire band is converted to all digital operation. Notethat IBOC frequency reuse is superior to FM, which requires at least a25-30 dB signal differential. IBOC only requires a 4-6 dB signaldifferential, allowing for closely spaced IBOC stations and makingbetter use of the FM band. This demonstrates the spectral efficiency ofthe all-digital IBOC signal standard.

The all-digital IBOC modes proposed by iBiquity (MS1-4) so far are notimplemented in broadcast transmitters or receivers. The current systemis backward compatible since it is built upon existing modulator andreceiver technology widely deployed today. Multiple exgine IBOCmodulators (latest 4_(th) generation) and other IBOC transmissioncomponents can be executed on one or more exciter hardware platformsprovided on one or more CPUs, DSPs, and/or FPGAs. All presentlyimplemented IBOC service modes may be used with the current multiplexingsystem and not all stations in the multiplex need to have the sameservice mode. Future service modes, such as single sideband modes arealso expected to be applicable.

Potential application areas for the multiplexing system include:

-   -   HD conversion by leasing an audio stream on the multiplex        -   Channel operation can be financed via ad insertion not            possible using today's simulcast    -   Moving AM stations to the FM band as proposed by some countries        such as Mexico    -   Netcasters with a large number of audio streams can place the        most popular streams on air    -   LPFM operators may opt to use a side channel on an HD multiplex        rather than their own signal

Placing multiple IBOC stations onto a single transmission system makesbetter use of the IBOC transmitter as the transmission power costdecreases per Watt with the size of the broadcast transmitter.Furthermore, using a single antenna system to broadcast the multiplexhelps the receiver separate each individual IBOC station as the “firstadjacent” desired/undesired (D/U) ratio is fixed.

It is conceivable to design a 600 kHz, or more, receiver that is able todecode the entire multiplex at once using 2 or 3 standard IBOC signaldemodulators and extract all the audio and data services at once. Sinceall signals in a multiplex exhibit synchronized symbol timing only asingle symbol tracking loop is required provided the receiver isinformed all signals are part of the same multiplex. The tracking loopcan either look at a one or more stations and extend the symbol timingacross the other stations or the tracking loop can look at thecombination of all stations and derive the symbol timing. A singlechannel estimator and sub-carrier demodulator can be used across thewider bandwidth to extract all the audio and data services at once. Whenlooking at the constellation in the frequency domain, the receiver mustcorrect for the phase offset in the same method as described in thisdocument for iterative peak reduction. This is obviously distinct fromindependent receivers performing this operation tuned to their ownrespective stations and later bonding the data. Using a single widerbandwidth receiver embodiment promises significant hardware resourcesavings. A receiver implementing the above technique is depicted in FIG.12. With an audio offering of up to 15 audio streams to choose from, asmart receiver could build a customized audio playlist based on listenerpreferences. An electronic program guide, channel list and alternatefrequency/stream system may be used to inform a listener of availableaudio streams. Combined with improved HD scan capability the listenermay be able to easily discover new audio and data offerings. Thedynamics of having a large number of audio streams under the control ofa single broadcast entity as provided by the current system may justifydeveloping and deploying electronic program guides to receivers andlisteners.

FIG. 12 depicts a multiplex receiver implementation. The receiver system1200 receives a radio frequency signal at a receiver antenna 1202. Thereceived signal may be processed by an automatic gain control 1204 inorder to provide a signal at appropriate levels for further processing.The signal comprises a combined multiplex signal 1206, which comprises aplurality of different station signals. As depicted, symbols of each ofthe stations in the multiplex signal 1206 are time aligned, and as sucha single symbol tracking loop 1208 is able to process the multiplexsignal to provide the symbols 1210 for all of the stations. The boundedsymbols may be processed by a pulse shaper 1212 in order to conditionthe signal. A wide bandwidth FFT 1214 is applied and the phase adjusted1216 a, 1216 b as required. The phase corrected signal is then used forchannel estimation 1218 and the data decoded and error corrected 1220for each of the stations present in the multiplex. The audio and dataservices 1222 carried by each station of the received multiplex areoutput from the receiver.

It is possible to input multiple independent stations into a singlecrest factor reduction engine, such as that described in HD PowerBoost,in order to peak reduce the entire multiplex. It is possible to simplyfrequency shift individual IBOC signals, combine them digitally or inanalog and use common amplification. Consider, however, that each IBOCsignal has a PAPR of at least 6 dB. Adding N stations together increasesthe PAPR according to the following formula:PAPR_(total)(dB)=10 log(N·PAPR_(single)(linear))=PAPR_(single)(dB)+10log N

In the example of 3 stations being added together, the resulting PAPRwould be given by:PAPR_(3 stations)(dB)=PAPR_(single)(dB)+10 log 3=6 dB+4.77 dB=10.77 dB

By combining the signals into a single PAPR reduction engine, an overallPAPR comparable to that of a single station can be obtained; effectivelythe signal energy spreads across the participating stations in themultiplex.

FIG. 2 shows a block diagram of N standard iBiquity supplied exginemodulators feeding a single peak-to-average-power reduction engine. Thefrequency shift shown in the diagram is conceptual and can also beperformed as a bin reassignment in the frequency domain. In this case,each IBOC symbol is first demodulated by removing the pulse shapingfunction and then transformed into the frequency domain using an FFT.Note that a true 100 kHz shift cannot occur as bins must be shifted anintegral number of sub carrier spacings of 363.4 Hz in order to preservethe IBOC constellation across the multiplex. The closest shift is 275frequency bins either way for 100 kHz. This translates into an effectiveshift frequency of 99.928 kHz. At 87.7 MHz a frequency error of 72 Hztranslates into an error of 0.82 ppm. This is within the tolerance ofthe IBOC signal specification of 1 ppm. It does mean that the maintransmitter frequency accuracy should be better than 0.2 ppm likelyrequiring GPS synchronization of the transmitter.

The following calculations are shown for the IBOC modulator sample rateof 744187.5 Hz. The same calculations can be performed at oversampledrates. An integer multiple may be used in order to extend the multiplexwith more carriers.

The effect of applying a continuous frequency shift on a cyclic OFDM forthe off frequency stations has the effect of a symbol-to-symbol rotatingphase offset. This is due to the fact that the continuous frequencyshift keeps rolling past the 2048 IBOC samples over the added 112 guardinterval samples. If the signal is to be presented as independent IBOCsidebands to the receiver, this effect must be maintained—this isdifferent than simply having more OFDM carriers. Failing to do so, areceiver may incorrectly interpret this effect as a gradual delay slipdue to a sampling frequency offset. If the receiver takes action on thisarbitrary error, the correction can introduce noise and bit errorsleading to a loss in signal quality.

For the 275 bin frequency shift described above, this means that thecontinuous complex frequency of 275/2048*744187.5 Hz=99,928 Hz keepsrunning for the 112 samples of the guard interval at 744187.5 Hz. Thisadds a fixed phase offset at the start of the next symbol of 94.5radians. The modulus of 2*pi may be taken, which leaves asymbol-to-symbol phase correction of 0.2454 radians as expressed by:

${{{phase}\mspace{14mu}{correct}} = {\left( {112*\frac{bins}{2048}*2\pi} \right){{mod}\left( {2\pi} \right)}}};{{bins} = {{275@100}\mspace{14mu}{kHz}}}$

Preserving this aspect means that the PAPR reduction engine does nothave fixed constellations to work with for the off frequency stations.This prevents the algorithm from performing the frequency domaincorrection steps outlined in the HD PowerBoost description, includingcorrecting the noise in the unused frequency bins, limiting the MER inthe data carriers and correcting the phase of the reference carriers.

Modifying the PAPR reduction algorithm is shown in FIG. 3. The PAPRreduction depicted in FIG. 3 may, for example, replace block 192 in FIG.2 of HD PowerBoost. It utilizes a running phase accumulator for everyshifted individual station that gets updated after every symbol by theoffset computed above. The accumulator is subtracted for a negativeshift and added for a positive shift. Alternatively, the phase shift canbe multiplied into the frequency bin using e^(j phase acc) and thisscalar can be updated by multiplying the phase correction value into therunning accumulator as shown here:e ^(e j phase acc) =e ^(j phase acc) *e ^(j phase correct)

To undo the phase shift prior to the frequency domain corrections asdescribed in HD PowerBoost (noise bins, data carrier and referencecarrier MER), the phase angle of the running accumulator is simplynegated. In the implementation, the HD PowerBoost engine separates eachfrequency bin as belonging to one of the interleaved stations and applythe negated phase accumulator corresponding to the station.

It is important to note that the phase correction, while required, doesnot invalidate the orthogonality property of the OFDM signal. Allcarriers of each single station are orthogonal to the carriers in theirneighboring stations.∫_(sym start) ^(sym end)carrierA(t)carrierB(t)dt=0

In FIG. 1, the 3 exemplary stations were interleaved in anA_(L)B_(L)C_(L)A_(U)B_(U)C_(U) pattern, where each individual station isassigned a letter. L and U subscripts identify the lower and uppersidebands respectively. A space without a sideband is marked by anunderscore.

Interleaving IBOC signals has been discussed in the context of IBOCfrequency planning for all-digital IBOC transmission, having theinterleaved multiplex emitted from a single transmitter is novel. Forfrequency planning, the interleaving pattern typically is A_(L)_(_)B_(L)A_(U)C_(L)B_(U)D_(L)C_(U)E_(L)D_(U) _(_)E_(U), as depicted inFIG. 11, creating a continuous chain of individual stations across theFM band. A new interleaving pattern of ABCABC may be used to create apacked 600 kHz signal in order to be able to pass it through a standardexciter up-conversion chain. A 1 MHz signal variant could be created asA_(L) _(_)B_(L)A_(U)C_(L)B_(U)D_(L)C_(U) _(_)D_(U), as depicted in FIG.13, a 1.2 MHz variant can beA_(L)B_(L)C_(L)A_(U)B_(U)C_(U)D_(L)E_(L)F_(L)D_(U)E_(U)F_(U), asdepicted in FIG. 9. Further combinations are possible when consideringsingle sideband stations as part of the multiplex. For example, the 1MHz solution can be reduced to 600 kHz by turning off the outersidebands (B_(L)A_(U)C_(L)B_(U)D_(L)C_(U), as depicted in FIG. 8). Thisprovides a solution with 4 stations at 200 kHz channel spacing. FIGS. 1and 4-11 and 13 show spectra representing just a few of the possiblesignal configurations.

Further station interleaving combinations using more and more bandwidthare possible; however, at some point becomes impractical in a typicalbroadcast transmitter with more demanding baseband envelopes anddiminishing resolution per station.

The system described herein uses a single transmitter to broadcast 2 ormore independent IBOC signals (or other digital signal types). Forexample, 3 stations interleaved as A_(L)B_(L)C_(L)A_(U)B_(U)C_(U),depicted in FIG. 1. Further, 2 stations may be interleaved as A_(L)_(_)C_(L)A_(U) _(_)C_(U), depicted in FIG. 7 The system may providereduced bandwidth modes using single sidebands, such asB_(L)C_(L)A_(L)B_(U), depicted in FIG. 5, or C_(L)A_(L). The sidebandlevels may be adjustable (including sideband off—single sideband). TheIBOC service modes are independently configurable such as MP1,2,3 forhybrid and MP5,6 for all digital. Future modes will likely work,including new single sideband modes.

The system described herein provides peak reduction, pre-correction,amplification and transmission of the interleaved multiplex, which maybe provided based on the peak reduction described in HD PowerBoost orsimilar algorithms for example algorithms based on the patent family ofKroeger and Shastri (WO2001015402 A1 and derivatives). Other methods,such as clip and filter may be used. With the system described herein,PAPR stays about constant with addition of carriers. The transmittedsignal provides a receiver the appearance of three independent stationsfrom a single OFDM modulator. The system corrects the symbol-to-symbolphase rotation resulting from the frequency shift in order to be able tocorrect the constellation of carriers within the multiplex. The systemmay utilize independent, yet synchronized orthogonal modulators thathave synchronized symbol timing, perfect frequency and standard carrierspacing. Utilizing the frequency spectrum of an oversampled IBOC signalmay further extend the multiplex interleaved pattern, for example a 1MHz solution may be interleaved as A_(L)_(_)B_(L)A_(U)C_(L)B_(U)D_(L)C_(U) _(_)D_(U), as depicted in FIG. 13,while a 1.2 MHz solution may be interleaved asA_(L)B_(L)C_(L)A_(U)B_(U)C_(U)D_(L)E_(L)F_(L)D_(U)E_(U)F_(U), as shownin FIG. 9. Other interleave patterns may be provided. The independentsignals may be added via frequency shift and addition only, without theuse of FFTs. The system may provide SFN implementations of all or justpart of the IBOC multiplex.

Although the above describes the system with regard to multiplexing IBOCstations, the system may be applied to other OFDM signal types such asDRM+, or China Digital Radio.

The system may include optional FM carriers to allow receivers to scanfor the station. The phase of multiple FM carriers may be controlled toavoid excessive peaks through FM carrier addition. Narrow band FMmodulation may be used to provide a stream instructing the analog FMlistener to tune in via a digital radio. A standard FM signal may bemaintained as part of the larger multiplex. For example, a pattern ofA_(U)B_(L)FFB_(U)C_(L), where the F denotes the standard FM signal, canbe created providing more IBOC bandwidth to a standard hybrid FM+IBOCstation as used with HD PowerBoost. Channel combining of conventionalhybrid FM+IBOC stations is an application for maintaining the FM carrierin conjunction with this concept. Multiple frequency shifted IBOCsignals and peak reduced with the described method can be broadcast viaa single transmitter and antenna system. The corresponding FM carriersof the stations can be broadcast using a channel combiner on oneantenna. Using the HD PowerBoost concepts allows one FM carrier to bemaintained as part of the larger multiplex, as shown in FIG. 10. Havingmultiple FM carriers in the multiplex is possible, but not practical asthe FM carriers will beat with one-another and create excessive peaks.It can be appreciated that the FM signal, treated as per reference only,can be substituted with any unrelated signal type that is to bemaintained. Just like HD PowerBoost clipping noise can be treateddifferently on the FM signal, that is maintained as is, and the IBOCsignal that is operated on.

It may be possible to synchronize multiple exciters together to allowthe multiplex of the stations to be spread across multiple transmitters.An electronic program guide, channel lists and alternate frequencyinformation may be provided to a receiver tuned into a single (IBOC)signal in order to provide the receiver with information about the otherstations on the same multiplex. The receiver may no longer need to scanfor the other stations on the multiplex to be discovered.

Independent single station modulator components (e.g. exgine, importer,exporter) can be implemented on a single or on multiple hardwareplatforms, such as CPUs, FPGAs, or similar.

A receiver with a 600 kHz+ bandwidth may simultaneously receive allstreams and “channel bond” the data, which may provide variousapplications including personal radio and/or providing conditionalaccess to some audio/data services or just the enhanced audio. Thereceiver may use 2 or more existing IBOC demodulators.

FIG. 4 depicts 3 interleaved stations. FIG. 5 depicts a 400 kHz modecomposed of one station using dual sidebands and 2 stations using singlesideband modes. A 200 kHz mode is also possible by turning off the outersets of carriers leaving only two stations each using a single sidebandthat could either be an upper or lower sideband. FIG. 6 depicts anunmodulated FM Carrier with three sets of IBOC signals. An unmodulatedcarrier can be used to allow receivers to scan for the station. Up to90% of the broadcast power is allocated to the FM carrier. Each singlestation can maintain its own carrier. The beating of the 3 carriers andthe resulting signal peaks can be addressed in the crest factorreduction engine by cancelling with the IBOC carriers. Wide deploymentof improved HD scan capability is seen as a possible solution to thescanning problem of having the interleaved stations. It may not bedesirable to have analog receivers tune in to this all digital signal,so it may be desirable to turn off the FM carrier in the first place. Ifthe carrier is present, it may be possible to add very low levelmodulation instructing the listener of an analog radio to tune in usingan HD radio receiver. FIG. 7 depicts a dual station configuration. FIG.1 depicts a 3 station configuration. Four stations are possible if thecenter ones are single sideband as shown in FIG. 8.

The hardware, software, firmware and combinations thereof providing theabove described functionality may reside in the same physical systems,or may be distributed in multiple devices and/or systems.

Although specific embodiments are described herein, it will beappreciated that modifications may be made to the embodiments withoutdeparting from the scope of the current teachings. Accordingly, thescope of the appended claims should not be limited by the specificembodiments set forth, but should be given the broadest interpretationconsistent with the teachings of the description as a whole.

What is claimed is:
 1. A digital broadcast system comprising: a firstinput component configured to receive a first plurality of stationsignals and frequency-shift the first plurality of station signals toprovide a first multiplex of the first plurality of station signals; afirst power reduction component configured to peak reduce the firstmultiplex and provide a corresponding first output signal; and a firsttransmitter component configured to transmit the first output signal,wherein the first power reduction component is configured to peak reducethe first multiplex based on a correction algorithm that controlsin-band and out-of-band noise from the peak reduction and operates on asignal in a frequency domain, and wherein the first power reductioncomponent is configured to undo a symbol-to-symbol rotating phase offsetprior to frequency domain corrections and to reapply thesymbol-to-symbol rotating phase offset following the frequency domaincorrections.
 2. The digital broadcast system of claim 1, wherein two ormore station signals of the first plurality of station signals compriseIn-band-On-Channel (IBOC) station signals.
 3. The digital broadcastsystem of claim 2, wherein two or more station signals of the firstplurality of station signals in the first multiplex are interleaved infrequency.
 4. The digital broadcast system of claim 2, wherein servicemodes and sideband levels of the IBOC station signals are independentlyconfigurable.
 5. The digital broadcast system of claim 1, wherein eachstation signals of the first plurality of station signals in the firstmultiplex is independently adjustable in power.
 6. The digital broadcastsystem of claim 1, wherein the first plurality of station signals isproduced by multiple independent modulators.
 7. The digital broadcastsystem of claim 6, wherein each modulator of the multiple independentmodulators has synchronized symbol timing.
 8. The digital broadcastsystem of claim 7, wherein each modulator of the multiple independentmodulators produces an orthogonal frequency-division multiplexing (OFDM)signal with subcarriers perfectly spaced in frequency to all carriers inthe first multiplex of the first plurality of station signals by astandard carrier spacing as defined by an employed OFDM signal standard.9. The digital broadcast system of claim 8, wherein frequency shiftapplied to each station signal of the first plurality of station signalscomprises an integral part of the standard carrier spacing.
 10. Thedigital broadcast system of claim 1, wherein one or more station signalsof the first plurality of station signals use a single frequency networkimplementation.
 11. The digital broadcast system of claim 1, wherein thefirst output signal includes one or more frequency modulation (FM)carriers.
 12. The digital broadcast system of claim 11, wherein the oneor more FM carriers allow a receiver to scan for one or more stationsignals of the first plurality of station signals.
 13. The digitalbroadcast system of claim 12, wherein the one or more FM carriers aremodulated to provide a signal instructing a listener to tune in via adigital radio.
 14. The digital broadcast system of claim 1, furthercomprising: a second input component configured to receive a secondplurality of station signals and frequency-shift the second plurality ofstation signals to provide a second multiplex of the second plurality ofstation signals that has synchronized symbol timing with the firstmultiplex; and a second power reduction component configured to peakreduce the second multiplex and provide a corresponding second outputsignal.
 15. The digital broadcast system of claim 14, furthercomprising: a signal combiner configured to combine the first outputsignal with the second output signal for transmission by the firsttransmitter component; and/or a second transmitter component configuredto transmit the second output signal.
 16. The digital broadcast systemof claim 1, wherein the first plurality of station signals comprisesorthogonal frequency-division multiplexing (OFDM) station signals.